The present invention relates to an air duct connected to the intake system of an engine, and more particularly, to an air duct that has a function to trap fuel vapor leaking from the engine intake system when the engine is not running.
Conventionally, for example, Japanese Laid-Open Patent Publication Nos. 2006-348834 (first prior art), 2001-336454 (second prior art), 2006-226123 (third prior art) each disclose such a fuel vapor trapping device.
The device of the first prior art includes an air cleaner and a filter element, which is arranged in the housing of the air cleaner so as to intersect an air passage. In the housing of the air cleaner, a fuel adsorbing member is located downstream in air flow of the filter element. Further, the fuel adsorbing member is arranged to intersect the air passage. The fuel adsorbing member includes a holder sheet and a cover sheet. The holder sheet includes a sheet base, which is made of nonwoven fabric and contains granular activated carbon. The cover sheet is made of nonwoven fabric and covers the holder sheet.
The device of the second prior art also includes an air cleaner and a filter element, which is arranged in the housing of the air cleaner so as to intersect an air passage. A plurality of reinforcing ribs are formed on the inner wall surface of the air cleaner, so as to be located downstream in the air flow from the filter element. Fuel adsorbent, which is formed by solidifying activated carbon powder with binder, is embedded between the reinforcing ribs.
Further, the device of the third prior art includes an air duct located between an air cleaner and an engine. Fuel adsorbent is provided on a part of the inner wall surface of the air duct. The fuel adsorbent is made of a woven fabric duct of activated carbon fibers.
The devices of these prior arts have the following drawbacks. That is, the device of the first prior art includes a fuel adsorbing member that is located in the housing of an air cleaner so as to intersect an air passage. Thus, the pressure loss of intake air flow is great while the engine is running, and the intake efficiency of the engine is lowered.
In contrast, in the devices of the second and third prior arts, fuel adsorbent is provided on the inner wall surface of the air cleaner housing or on the inner wall surface of the air duct. Therefore, unlike the device of the first prior art, the pressure loss of intake air flow is hardly increased. However, fuel adsorbent provided on the inner wall surface of an air cleaner housing or of an air duct has lower adsorbing and desorbing performances than a fuel adsorbing member arranged to intersect an air passage as in the device of the first prior art.
In other words, while the engine is running, intake air is likely to flow along an area about the center axis in the housing or in the air duct, and less likely to reach the fuel adsorbent on the inner wall surface in the housing or in the air duct. Therefore, fuel vapor that has already been adsorbed and trapped by the fuel adsorbent is hardly desorbed from the fuel adsorbent. As a result, the fuel adsorbent has a low fuel adsorbing performance.